You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <alltransclusions gatcontinue="Card|4773" />
  </query-continue>
  <query>
    <pages>
      <page pageid="3089" ns="0" title="Custom Cards" />
      <page pageid="3130" ns="0" title="Custom Enchants" />
      <page pageid="4764" ns="0" title="Clown Card" />
      <page pageid="4765" ns="0" title="Professor Card" />
      <page pageid="4766" ns="0" title="Champion Card" />
      <page pageid="4767" ns="0" title="Creator Card" />
      <page pageid="4769" ns="0" title="Stalker Card" />
      <page pageid="4770" ns="0" title="Paladin Card" />
      <page pageid="4771" ns="0" title="Gypsy Card" />
      <page pageid="4772" ns="0" title="Alphoccio Card" />
    </pages>
  </query>
</api>